1. Technical Field of the Invention
The present invention relates to imagable articles, and coating compositions therefor. In particular the present invention relates to imagable articles, and coating compositions comprising positive working, heat sensitive materials including heat-labile moieties. The invention also extends to methods of manufacturing imagable articles.
2. Background to the Invention
Imagable articles, such as lithographic printing form precursors, electronic part precursors and mask precursors conventionally comprise a substrate onto which is coated a film-forming, radiation sensitive composition. After image-wise exposure of the precursor to radiation of suitable wavelength, and development of the imaged precursor, an imaged article is produced.
Positive working imagable articles have a radiation sensitive coating, which, when image-wise exposed to radiation of a sensitive wavelength, becomes more soluble in a developer liquid in the exposed areas, than in areas not exposed. On development, the exposed areas of the coated composition are removed, leaving an image formed from the remaining areas.
The coatings are normally laid down as solutions in organic solvents, which are removed by evaporation. Conventionally, ultra-violet or visible radiation has been employed to image compositions sensitive to radiation of those wavelengths. Recently infra-red (IR) radiation has become of interest.
In many systems the compositions contain IR absorbers which convert IR radiation to heat, and it is the heat which is the direct cause of the increase in the solubility of the compositions. A heated body delivering heat conductively to corresponding compositions not containing IR absorbers can likewise effect a solubility change. A suitable IR radiation source is an IR laser digitally controlled to produce the required pattern of heated areas. Thus, these compositions are suitable for advanced xe2x80x9cComputer-to-Platexe2x80x9d (CTP) techniques. Some compositions, which are not additionally sensitive to ultra-violet or visible radiation offer the advantage, over traditional photosensitive recording compositions, that they do not need to be handled in a dark room, or under ultra-violet safelighting conditions, but can simply be handled in ordinary light.
There are many examples of positive working compositions which are sensitive to UV or visible radiation, and which are used in coatings on imagable articles. Many such positive working compositions utilize polymers in conjunction with acid-cleavable moieties and acid generating compounds. In these systems the composition is exposed to UV or visible radiation which causes generation of and acid, which acid then effects cleavage of the acid-cleavable group to render the exposed polymer more soluble in a developer than the unexposed polymer.
Examples of UV-sensitive systems employing resist materials comprising a polymeric binder having an acid-cleavable group and an acid generating moiety include those described in U.S. Pat. Nos. 5,073,474, U.S. 5,882,835, U.S. 5,066,566, U.S. 4,491,628, U.S. 4,883,740, U.S. 6,033,827, U.S. 5,827,634, U.S. 5,759,750, U.S. 5,350,660, U.S. 5,075,199, JP10055067, U.S. Pat. Nos. 5,994,023, and 5,102,771.
Examples of UV-sensitive systems employing polymeric binders having acid cleavable groups, and acid generating compounds also include the systems disclosed in SPIE Vol. 920, xe2x80x9cAdvances in Resist Technology and Processing Vxe2x80x9d, 1988, pages 60-63, and pages 42-50.
In many cases the acid generators used are relatively expensive and may be unstable with regards to premature acid generation.
Initial studies have been undertaken to assess the effect of heat on deprotection of tert-butoxycarbonyl (t-BOC) protected polymeric binder compositions, such as in xe2x80x9cPhotoreactive Polymersxe2x80x9d (A. Reiser) [1998] pages 279-283. The thermodynamic behaviour of dissolution of polymeric binders including t-BOC based inhibitor compounds has also been investigated, as in xe2x80x9cEffect of Substitutent Groups of Phenol Derivative Dissolution Inhibitors on Inhibition Efficiencyxe2x80x9d, SPIE Vol. 2724 (1996).
It is an aim of preferred embodiments of the present invention to overcome or mitigate at least some of the problems associated with the prior art discussed above or other problems whether expressly disclosed hereinabove or not.
It is a further aim of preferred embodiments of the present invention to provide an effective thermal imaging technology.
Applicant has surprisingly found that in imagable articles and compositions, certain heat-labile moieties can be used to protect hydroxyl group-containing polymers, wherein upon exposure to heat the heat-labile moiety is removed or a complex formed by the heat-labile moiety and polymer is broken, whereupon the composition including the hydroxyl group-containing polymer exhibits increased developer solubility. Using such systems negates the need for acid generating systems or exposure to UV or visible radiation. Thus the systems of the present invention may be relatively inexpensive, and have improved handling characteristics in ambient light. Furthermore, the developer solubility differential between exposed and non-exposed compositions is good.
Applicant has further found that the use of a radiation absorbing compound, capable of absorbing radiation and converting it to heat, in the imagable and imaged articles and compositions of the present invention, serves to further increase their beneficial characteristics.
According to a first aspect of the present invention there is provided a thermally imagable article comprising a substrate on which is coated a positive working heat-sensitive composition comprising a hydroxyl group-containing polymer and a heat-labile moiety which decreases the developer solubility of the composition as compared to the developer solubility of the composition without the heat-labile moiety, wherein the heat-sensitive composition does not comprise an acid generating moiety.
By acid generating moiety we mean a moiety capable of generating an acid on exposure of the imagable article to heat and/or radiation.
By heat-labile moiety we mean a moiety which is capable of undergoing separation from and/or deprotecting a parent material when exposed to heat. Deprotection according to this invention includes chemically cleaving or splitting the heat-labile moiety, or removing the heat-labile moiety from the parent material.
It is believed that exposure of the imagable article to heat causes removal or splitting of the heat-labile moiety or deprotection of the polyhydroxyl group-containing polymer, which causes an increase in the developer solubility of the composition.